In the last years, the need for highly accurate, quick, reliable filter-tuning techniques has become acute. A field of high interest is represented by high performance resonators and filters, especially medium-to-high power miniature filters with wide tunable bandwidth. These kind of devices need to have very small dimensions, in order to fit inside high-performance planar filters, such as micromachined stripline and High Temperature Superconductor (HTS) multi-pole filters.
Micromachining is a term commonly used to describe chemical etch processes that selectively remove certain materials in certain places, as opposed to standard machining techniques that mechanically remove material. Due to the very tight tolerances that can be maintained in a chemical etch, very small and highly accurate features can be produced, hence the term “micro” machining. In addition, many substrates that are commonly used in electronic circuit applications (eg silicon, glass) are not easily machined using standard techniques due to the possibility of material fracture.
Devices of this kind that are appropriate for tunable filters and phase shifters are microelectromechanical (MEM) capacitors and capacitive MEM switches. However, variable capacitors thus fabricated that rely on electrostatic actuation are capable of only a 33% capacitance change due to the so called “snap down” phenomenon, as better explained in the following. The continuous tunability of these devices in resonator/filter center frequency is less than 10%.
Other filter-tuning approaches are known, such as bulk ferroelectric materials, varactor diodes, and Barium Strontium Titanate (BST) thin films. See for example U.S. Pat. No. 5,990,766 (bulk ferroelectric tuning), U.S. Pat. No. 4,468,644 (varactor diode tuning) and U.S. Pat. No. 5,877,123 (thin film tuning). However, also these devices have disadvantages, principally due to a low third-order intercept point (IP3) and to high insertion loss. The disadvantage with tunable filters having a low IP3 is that they have very limited power-handling capability, so that they are not usable for example, in radar systems as preselectors. Further, another disadvantage of bandpass filters with high insertion loss is that they do not have a narrow bandwidth. In addition, bulk ferroelectric material requires high applied voltage to tune the device, and is expensive.